Swing objective lens

ABSTRACT

A scanning electron microscope (SEM) with a swing objective lens (SOL) reduces the off-aberrations to enhance the image resolution, and extends the e-beam scanning angle. The scanning electron microscope comprises a charged particle source, an accelerating electrode, and a swing objective lens system including a pre-deflection unit, a swing deflection unit and an objective lens, all of them are rotationally symmetric with respect to an optical axis. The upper inner-face of the swing deflection unit is tilted an angle θ to the outer of the SEM and its lower inner-face is parallel to the optical axis. A distribution for a first and second focusing field of the swing objective lens is provided to limit the off-aberrations and can be performed by a single swing deflection unit. Preferably, the two focusing fields are overlapped by each other at least 80 percent.

CLAIM OF PRIORITY

This application claims the benefit of priority of U.S. provisionalapplication No. 62/089,547 entitled to inventor Shuai Li, filed Dec. 9,2014 and entitled “Swing Objective Lens System for Tilting ElectronBeam”, the entire disclosures of which are incorporated herein byreference.

FIELD OF THE INVENTION

The invention relates to swing objective lens. The invention alsorelates to a method for tilting an electron beam. However, it would berecognized that the invention has a much broader range of applicability.

BACKGROUND OF THE INVENTION

The following description and examples are not admitted to be prior artby virtue of their inclusion in this section.

Undoubtedly, compared to any other technology or knowledge,semiconductor devices not only impact nowadays society but alsoinfluence our daily life. Although it can be traced to two centuriesago, for example Alessandro Volta in 18th century and Michael Faraday in19^(th) century, the history of semiconductor development indeedinfluences mankind in commercial semiconductor devices is 20^(th)century. In the first, vacuum tube transistor is replaced by thesemiconductor devices which mainly include BJT (Bipolar JunctionTransistor) and MOSFET (Metal-Oxide-Semiconductor Field EffectTransistor), and then the semiconductor devices are minimized intointegrated circuits. No matter digital logic circuit device, analogcircuit device or communication devices, these semiconductor devices canbe fabricated on silicon-based substrate or semiconductor compoundsubstrate. Another semiconductor devices are photoelectronic deviceswhich mostly include LED (Light Emitting Diode), LD (LASER Diode) orphotovoltaic cell base on photoelectric effect. Currently, digitalelectronic devices based on MOSFET fabricated in silicon substrate arecommercially the most significant, and the applications of the devicesare processors and memory devices.

Fabrication processes for manufacturing ICs in the silicon substrateinclude cleaning process, oxidation and thermal process,ion-implementation process, thin film deposition, lithography, etchingprocess and CMP (Chemical Mechanical Polishing) process. By thecombination of the above processes, when all electronic devices areformed in the substrate, and then followed by metallization process toelectric connect all electronic devices, a specific application device,such as CPU, ASIC, FPGA, DRAM, or Flash, can be produced. With thetechnology progress of semiconductor process, the smaller width of anelectronic device followed by Moore's law which means transistors aredoubled every 24 months, the more devices in one wafer can be fabricatedto cost down.

The semiconductor fabrication processes include ion implantationprocess, thermal process, thin film deposition process, etching process,CMP (Chemical Mechanical Polishing) process, lithography, and cleaningprocess. And they will be briefed hereinafter.

Ion-implantation process will direct group III or group V atomsimplanted into silicon substrate to alter local electric conductivitysuch that some regions are positive conductivity and some regions arenegative conductivity. Phosphorus or Arsenic atoms are usually used forthe negative conductivity, while Boron atom is usually used for thepositive conductivity.

Thermal process provides formation of thermal oxide layer and annealingfor drive-in after ion-implantation. In the present art, RTP (RapidThermal Process) is popular instead of conventional thermal process infurnace. It includes RTO (Rapid Thermal Oxidation) and RTA (RapidThermal Annealing) to respectively form silicon oxide and repair latticedamages after ion-implantation such that single crystal structure can berecovered and dopant can be activated.

Thin film deposition process includes PVD (Physical Vapor phaseDeposition) and CVD (Chemical Vapor phase Deposition) to form several toseveral tens thin film layers with variant materials and thicknesses onsilicon substrate. Metal layers, formed on a substrate, always provideelectric interconnections among devices, while dielectric layer providesisolation between metal layers. Chemical reactions in vapor phase,happened to form thin films in CVD, include MOCVD (Metal-Organic CVD),APCVD (Atmosphere Pressure CVD), LPCVD (Low Pressure CVD), HPCVD (HybridPhysical CVD), RTCVD (Rapid Thermal CVD), HDPCVD (High Density PlasmaCVD), and PECVD (Plasma Enhanced CVD). Thin films forms by CVD usuallyinclude silicon oxide, silicon nitride, polysil icon, metal tungsten,metal aluminum, and metal titanium nitride. Metal target are heated orbombarded in vacuum such that atoms on the metal target can betransferred to substrate surface to form thin film in PVD, whichincludes evaporation and sputtering. Metal thin films, such as aluminum,titanium, or alloy thereof, are always formed by using PVD. Qualitycontrol of the thin film is critical to IC process, so thin film processmust be monitored throughout the procedure to reflect abnormal, suchthat thickness uniformity and defect and be avoided.

Etching process, which includes wet etch and dry etch, is to removematerial. In the semiconductor process, patterns on a reticle can betransferred to a thin film by using etch process. Wet etching isisotropic by reacting etchant to selective material, and etched profilealways reveals bowl-like shape. Dry etch is popular and anisotropic byreacting plasma in an external electric field with the selectivematerial, and etched profile will reveal vertical-like shape.

CMP is another method to remove material, which introduce slurry betweenpublish pad and wafer with chemical and mechanical reactions to achievewhole wafer planation, such that thin films in the following process canbe formed better. Silicon oxide layer, metal layer and polysilicon layerare most applied in the CMP process.

Lithography process, also named photo-lithography process, is the mostcritical in the semiconductor process, which includes PR (photo Resist)layer coating, soft bake, exposure, development, hard bake, and ashafter etching process. The PR can be selectively removed throughexposure and development, and circuit patterns can be transferred to aspecific material. When the semiconductor process continues shrinking,available RET (Resolution Enhancement technology), such as OPC (OpticalProximity Correction), immersion lithography, and EUV (ExtremeUltraviolet lithography), are applied.

Cleaning process must be processed after all other process recited aboveto avoid uninvited particles or residues to impact device quality, whichincludes rinsing wafer by DI (De-Ionized) water and drying the wafer.Ultrasonic agitation can be applied in the cleaning process. Thisprocess will clean out all pollutions, such as particles, organicmatter, inorganic matter, metal ions.

Defects are inevitably generated in the semiconductor process, whichwill greatly impact device performance, even failure. Device yield isthus impacted and cost is raised. Current defects can be classified intosystematic defects and random defects in general. On the one hand,system defects infer defects will be found repeatedly and systematicallyin wafers, in which defect patterns can be used as reference inclassification to determine root cause of which process incurs suchdefects. In order to increase semiconductor process yield, it iscritical to enhance yield by monitoring, such as by using SEM (ScanningElectron Microscope), systematic defects highly appeared regions in massproduction process to real time eliminate systematic defects. On theother hand, the non-systematic defects, random particle defects, arerandom residues left in wafers. Distributions and characteristicprofiles are important references to distinguish systematic defects fromnon-systematic defects.

More specifically, systematic defects can be classified as reticleerrors in alignment or machine offset, process mistakes incurred byrecipes or materials, prober damages in wafer probing, scratches onwafer surface, and wafer edge effect of topography incurred fromnon-uniformity of PR coating or thermal stress.

The corresponding defects are recited hereinafter in brief. Defectsincurred in lithographic process include PR residue defects due to PRdeteriorated or impurity, peeling defects, bridge defects, bubbledefects, and dummy pattern missing defects due to pattern shift. Defectsincurred in etching process include etching residue defects,over-etching defects and open circuit defect. Defects incurred in CMPprocess include slurry residue defects, dishing defects and erosiondefects due to variant polishing rates, scratched due to polishing.Further, when process nodes continue shrinking, new materials andprocesses will be introduced to inevitably incur new type defects. Forexample, because physical dimension of patterns are smaller than theoptical resolution of the applied lithographic wavelength of 193 nm, thecritical dimension exposed on wafers may incur offset. Thinning defectsare another inevitably incurred in the process node shrinking. In orderto reduce RC delay in multi-layered interconnection structures, low-kdielectric layer and cupper material are introduced. Cupper can't beetched and hence damascene process is introduced that metal is filledinto dielectric layer. Therefore, some other hidden defects are coveredunder layer, such as void defects, etching residue defects, over-etchingdefect, under layer particles, and via open incurred in theinterconnection process. Such hidden, crucial defects are too hard to beanalyzed and eliminated.

For the non-systematic defects are mainly random particles defectincurred from particles in air randomly fallen on the wafer, which arenot easy to be identified and resolved.

In order to enhance semiconductor process yield, defects have to beidentified as soon as possible to prevent from impact pouring out.Optical microscope is used in conventional optical inspection whichincludes bright field inspection and dark field inspection. Every die ona wafer is scanned by optical beam and images of every die are generatedand stored. A die-to-die compare is used to identify if there is anyabnormal or defect with locations and images thereof.

When semiconductor nodes continue shrinking, dimensions of defect shrinkalso. Unimportant small defects in previous now become criticaltherefore. It is a challenge to identify such small defects by usingconventional optical inspection tool and a new tool is necessary. Onemethod is to combine the operations of optical inspection and reviewSEM. Because of resolution, the optical inspection is not enough to meetrequirement of identifying defects, but a suspect region in blurredimages can be determined defect-like and reviewed by review SEM withhigh resolution. Thus defects can be identified and analyzed. Anothermethod is to illuminate dual beams on a wafer surface to obtaininterference patterns, and defect regions always have differentinterference pattern to that of the normal region. Thus, defects can beidentified and further analyzed by review SEM. In practice, defects mustbe identified first and locations of the defects are forward to reviewSEM with high resolution to analyze defects.

However, in sub-20 nanometer semiconductor node, optical inspection toolcan't reveal any pattern more, even by using interference method, andhence SEM is the only way to identify defects. Nevertheless, due to thedetected signal electrons in the SEM are secondary electrons, detectionduration inevitably retrogrades significantly compared to that ofoptical inspection. Hence, it is an important issue to fast identifydefects on a wafer by using SEM. An e-beam inspection tool, based onSEM, is currently best solution for defect inspection.

The e-beam inspection tool is to find or identify defects in thesemiconductor process, and relative to review SEM, a large FOV(Field-of-View) and large beam current are commercial means to enhanceinspection throughput. In order to obtain large FOV, a SORIL (SwingObjective Retarding Immersion Lens) system is applied commercially.Moreover, resolution is sometimes lowered, compared to review SEM,enough to capture defects.

The e-beam inspection tool is designed different from the review SEM.The review SEM is designed to known, identified defects or suspects ofdefect, so scan duration is long enough to analyze or review defects,and hence it can't process inspection. On the other hand, the e-beaminspection tool, with high scanning rate than the review SEM and highresolution than the optical inspection tool, can identify defects thatthe optical inspection tool in no way to capture.

Furthermore, in lithographic process, some particular patterns may havegreat possibility to incur defects, but won't incur them each time. Thedefects generated by these particular patterns even can't be modifiedthrough recipe tuning or modifying reticle directly. Such a kind ofpatterns is named hot spot, and must be monitored in-line process.

Applications of SEM, except yield management tool of e-beam inspectionand analysis tool of review SEM, may further be metrology tool insemiconductor manufacturing process; that is CD (criticalDimension)-SEM. CD-SEM will measure CD in a wafer with by line-scanningsample with moving stage to reveal process uniformity. Moreover, inorder to obtain exact dimension, resolution is very critical, and thuslow beam current must be applied.

Still another application of SEM is EBDW (E-Beam Direct Writer), ornamed EPL (E-beam Projection Lithography), still based on SEM. Purposesof EBDW are to expose a photoresist directly, and an etching step can beapplied to transfer patterns to a sample after the photoresist isdeveloped. In such a process, there is no reticle necessary, andpatterns are written directly on the sample. Because wavelength of ane-beam is superior small than an optical wavelength, finer patterns,such as nano scale resolution, can be easily obtained.

Nowadays semiconductor process node continues shrinking, a protrudinggate structure of FinFET is provided. In order to manage yield of thiskind of semiconductor device, side surface the gate is critical and hasto be inspected. Large tilting angle inspection is thus necessary.

BRIEF SUMMARY OF THE INVENTION

The object of this invention is to provide a SOL with large tiltingangle for inspecting a stereo structure. A stereo image can be obtained.

Accordingly, the invention provides a system for tilting a chargedparticle beam focused by an immersion objective lens, which comprises apre-lens deflector above and adjacent to a magnetic field generated bythe immersion objective lens, and a swing deflector located inside saidimmersion objective lens. The pre-lens deflector deflects the chargedparticle beam to a pre-determined angle, and the swing deflector swingsa magnetic lens generated by the immersion objective lens into thepre-determined angle, such that the charged particle beam is focused bythe swung magnetic lens and bombards a specimen with the pre-determinedangle.

The charged particle beam can be an electron beam.

The invention therefore provides a swing objective lens, which comprisesan immersion objective lens for focusing a charged particle beam on aspecimen, a pre-lens deflector above and adjacent to a magnetic lensgenerated by the immersion objective lens, and a swing deflector locatedinside said magnetic lens and providing an electrostatic field with afirst condition. The pre-lens deflecting the charged particle beam to apre-determined angle. When the first condition matches a secondcondition generated by the immersion objective lens and the swingdeflector, the magnetic lens is swung into the pre-determined angle.

The first condition is [φ″(z)+½φ″(z)(z−z₀)], and φ is an electricpotential produced by the swing deflector. The second condition is√{square root over (φ(z))}[B(z)+½B′(z)(z−z₀)], and B is the magneticfield generated by the immersion objective lens. The match can be thefirst condition mostly overlapped with the second condition. In apreferred embodiment, the match can be a first peak value of the firstcondition close to a second peak value of the second condition. In amost preferred embodiment, the match can be a first distribution of thefirst condition similar and close to a second distribution of the secondcondition.

The invention therefore provides a swing objective lens, which comprisesan immersion objective lens for focusing a charged particle beam on aspecimen, a pre-lens deflector above and adjacent to a magnetic lensgenerated by the immersion objective lens, and a swing deflector locatedinside said magnetic lens. The charged particle beam is along an opticalaxis. The pre-lens deflecting the charged particle beam to apre-determined angle. A first portion of an inner surface of the swingdeflector has an inclined angle to the optical axis and a second portionof the inner surface of the swing deflector being parallel to theoptical axis, wherein the magnetic lens is swung by the swing deflectorinto the pre-determined angle.

The swing deflector generates an electric potential φ with a firstcondition, [(φ′(z)+½φ″(z)(z−z₀)]. The immersion objective lens generatesa magnetic field B with a second condition, √{square root over(φ(z))}[B(z)+½B′(z)(z−z₀)]. The first condition matches the secondcondition. In a preferred embodiment, the match is a first distributionof the first condition similar and close to a second distribution of thesecond condition. The swing objective lens may further comprise ascanning deflector unit for scanning the charged particle beam on thespecimen. The swing objective lens may further comprise a retardelectrode below the immersion objective lens.

The present invention also provides a scanning electron microscope,which comprises an electron source for providing an electron beam alongan optical axis, a condenser lens for condensing the electron beam, aswing objective lens, and a detector for receiving signal electronsemanating from the specimen.

The present invention also provides a method for tilting a chargedparticle beam, which comprises steps of deflecting the charged particlebeam to a pre-determined angle, providing an immersion magnetic lens tothe charged particle beam such that the charged particle beam is focusedon a specimen, and providing a swinging electrostatic field to theimmersion magnetic lens such that the immersion magnetic lens is swungto the pre-determined angle, wherein the swing electric field has afirst condition match to a second condition generated by the immersionmagnetic lens and the swinging electrostatic field.

Other advantages of the present invention will become apparent from thefollowing description taken in conjunction with the accompanyingdrawings wherein are set forth, by way of illustration and example,certain embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages of the present invention may become apparent to thoseskilled in the art with the benefit of the following detaileddescription of the preferred embodiments and upon reference to theaccompanying drawings in which:

FIG. 1 is a diagram illustrating a conventional scanning electronmicroscope (SEM) with a deflection correction device to correct theoff-aberrations;

FIG. 2 is a diagram illustrating another conventional scanning electronmicroscope (SEM) with a compensation device and a movable stage tocorrect the off-aberrations;

FIG. 3a is a diagram of the first embodiment in the present invention,which illustrates a SORIL SEM;

FIG. 3b is a distribution of a first and second focusing field in thepresent invention, in which the two focusing fields are overlapped byeach other at least 80 percent;

FIG. 3c is a distribution of a first and second focusing field in thepresent invention, in which each space position of each maximum of thefirst and second focusing field is closed to each other;

FIG. 3d is a distribution of a first and second focusing field in thepresent invention, in which both of the focusing fields have similarspace distribution patterns of field intensities in the same region;

FIG. 3e is a diagram illustrates the shape, size and the arrangement ofthe swing deflector and magnetic lens, which are used to be modified toaccomplish SORIL;

FIG. 3f is a diagram illustrates the distribution, which changes frompositive to negative, of a first and second focusing field above aspecimen;

FIG. 4 is a diagram of the second embodiment in the present invention,which illustrates a SOL SEM.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and may herein be described in detail. Thedrawings may not be to scale. It should be understood, however, that thedrawings and detailed description thereto are not intended to limit theinvention to the particular form disclosed, but on the contrary, theintention is to cover all modifications, equivalents and alternativesfalling within the spirit and scope of the present invention as definedby the appended claims.

DESCRIPTION OF THE REFERENCE TERMS

An optical axis (or common axis) in the present invention is a centralaxis extending from the gun tip to the stage in a rotational symmetryelectron optical system.

A swing deflector unit in the present invention is a deflector, which islocated in a retarding field of a compound objective lens of an electronoptical system, used to swing a complex field of the compound objectivelens (i.e., generating a swing field).

A paraxial aberration in the present invention is an aberration which isoccurred in the area away from an optical axis, and usually is inducedwhen an electron beam is deflected by a lens.

An electrostatic lens in the present invention is an electro-opticallens formed by at least one electrode to focus and deflect an electronbeam.

A magnetic lens in the present invention is an electro-optical lensformed by a coil or a coil surrounded with a yoke to focus and deflectan electron beam.

A large field of view (LFOV) in the present invention is a large objectregion to be scanned by an electron beam, which can result in a scanspeed 144 times faster than the conventional one.

A swing objective lens (SOL) in the present invention is a lens near thespecimen, which can be swung by a swing deflector unit, for focusing anddeflecting an electron beam.

Landing energy (LE) in the present invention is an incident energy of acharged particle beam for a specimen, which can be changed by apotential different in an electro-optical tool.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term “specimen” generally refers to a wafer or anyother specimen on which defects of interest (DOI) may be located.Although the terms “specimen” and “wafer” are used interchangeablyherein, it is to be understood that embodiments described herein withrespect to a wafer may configured and/or used for any other specimen(e.g., a reticle, mask, or photomask).

As used herein, the term “wafer” generally refers to substrates formedof a semiconductor or non-semiconductor material. Examples of such asemiconductor or non-semiconductor material include, but are not limitedto, monocrystalline silicon, gallium arsenide, and indium phosphide.Such substrates may be commonly found and/or processed in semiconductorfabrication facilities.

The wafer may further include at least a portion of an integratedcircuit (IC), a thin-film head die, a micro-electro-mechanical system(MEMS) device, flat panel displays, magnetic heads, magnetic and opticalstorage media, other components that may include photonics andoptoelectronic devices such as lasers, waveguides and other passivecomponents processed on wafers, print heads, and bio-chip devicesprocessed on wafers.

Turning now to the drawings, it is noted that the figures are not drawnto scale. In particular, the scale of some of the elements of thefigures is greatly exaggerated to emphasize characteristics of theelements. It is also noted that the figures are not drawn to tie samescale. Elements shown in more than one figure that may be similarlyconfigured have been indicated using the same reference numerals.

In the drawings, relative dimensions of each component and among everycomponent may be exaggerated for clarity. Within the followingdescription of the drawings the same or like reference numbers refer tothe same or like components or entities, and only the differences withrespect to the individual embodiments are described.

Accordingly, while example embodiments of the invention are capable ofvarious modifications and alternative forms, embodiments thereof areshown by way of example in the drawings and will herein be described indetail. It should be understood, however, that there is no intent tolimit example embodiments of the invention to the particular formsdisclosed, but on the contrary, example embodiments of the invention areto cover all modifications, equivalents, and alternatives falling withinthe scope of the invention.

In this invention, “axial” means “in the optical axis direction of anapparatus, column or a device such as a lens”, while “radial” means “ina direction perpendicular to the optical axis”.

The present invention relates to an improved focusing and deflectionsystem of an e-beam inspection tool, in particular to a Swing objectivelens (SOL), for instance, a Swing Objective Retarding Immersion Lens(SORIL). Therein, the improved system can generate readily acompensation field to achieve a low paraxial aberration and a largescanning field.

In order to enhance the semiconductor product yield, it is needed forthe observation and examination of the IC patterned on the wafer anddefects occurred in the semiconductor fabrication processes. With theaggressive device scaling in modern IC technology, especially in 65 nmtechnology node below, the high aspect ratio (HAR) multi-levelsnanostructure is widely configured on the wafer. Therefore, there areappeared some latent defects that are difficult to be reviewed byconventional optical inspection tools. Moreover, when the technologynode comes to the 22 nm below, the optical inspection tool is almostincapable of examining the nanostructures. It is consequence that theSEM is the tool to achieve the above demands.

To further improve the speed of the observation and examination throughSEM, except the above-mentioned combination of the inspection SEM andthe review SEM, it also has to increase the inspection tool throughput.One of the solutions is the development of SEM for scanning the largerspecimen region, namely a large field of view (LFOV) of SEM.

Moreover, the low-voltage e-beam (less than 5 KeV) scan is the industrytrend. It has been known that a highly energized e-beam is used in SEMto improve the spatial resolution, but causes damage to specimens, evencauses undesirable charging of the specimen. Therefore, a SEM providinga low-energy e-beam scan (i.e., LVSEM), which decelerates thehigh-energy e-beam by a retarding field closed to the specimen, canavoid the reliability problem due to the damage of the specimen.

The low-aberration and LFOV scan of SEM has an objective lens systemincluding a focusing subsystem, and a deflection subsystem. Types of theobjective lens system are Moving Objective Lens (MOL), Variable AxisLens (VAL), Variable Axis Immersion Lens (VAIL), Swing Objective Lens(SOL), and Swing Objective Retarding Immersion Lens (SORIL), etc.Compared to the conventional SEM, which the deflection angle ranges from1° to 5°, the SEM with the larger scanning region has greater tiltedscan angle. As a result, the off-axis aberration becomes more severe.

In summary, it is a critical index for an e-beam inspection tool toprovide with a high-quality (high-resolution, low-aberration) images anda high throughput. However, the deteriorated off-axis aberration problemhas to be solved.

In order to reduce the paraxial aberration, many prior arts have putefforts into it. For example, U.S. Pat. Nos. 6,627,890, 7,531,799 andE.P. Pat. No. 2,378,537 have reported that an aberration correctiondevice arranged in the objective lens is employed to correct theaberration due to the deflected e-beam, which maintains the highresolution. However, the aberration corrector generally forms bymulti-electric pole plates or multi-magnetic pole plates, which havecomplex settings and need a complicated and high level power source tosupply voltages to the electric or magnetic plates. Please refer to FIG.1, which illustrates the embodiment of U.S. Pat. No. 6,627,890. Ane-beam is emitted from an electron source 101, then is deflected by adeflector 110, and enters an objective lens 106. An orbit 100 a of thee-beam is corrected by the deflection collector 112 to an orbit 100 bduring the objective lens 106 for lowering the off-aberration, and hitsa specimen 10. Additional examples of the deflection collection devicesare illustrated in U.S. Pat. Nos. 6,956,211 and 7,282,722 to Hitachi,Ltd., which disclose a system with a plurality of aberration correctionlenses to compensate the aberrations from an objective lens system.However, the arrangements of the collectors are complicated. In short,the above prior arts disclosed the deflection collection devices areused to generate a compensation field and correct the off-axisaberrations of e-beam produced as the e-beam passes through an objectivelens system.

In the theory, in order to generate a LFOV lens such as a SOL, thee-beam motion has to satisfy a trajectory equation (1), as shown in thefollowing. However, the serious off-axis aberrations usually exist forit is hard to satisfy the equation (1). It's known that the equation (1)is determined by the focusing field, the deflection field and the e-beamtrajectory in common, and is expressed as the following (by Y. Zhao andA. Khursheed, J. Vac. Sci. Technol. B 17, 2795 (1999)):

$\begin{matrix}{{\omega^{''} + {\frac{\varphi^{\prime}}{2\varphi}\omega^{\prime}} + \frac{\varphi^{''}}{4\varphi} - {i\sqrt{\frac{\eta}{2\varphi}}\left( {{B\;\omega^{\prime}} + {\frac{1}{2}B^{\prime}\omega}} \right)}} = {{- \frac{{VE}_{1}}{2\varphi}} + {\sqrt{\frac{\eta}{2\varphi}}{ID}_{1}}}} & (1)\end{matrix}$wherein ω=x+iy represents a radial deflection distance from Z-axis, (x,y) is the image plane, B and φ′ represent magnetic focusing field andelectric focusing field, respectively, E₁ and D₁ represent the firstorder differential functions of electric and magnetic deflection filed,respectively, V and I represent the complex deflection voltage andcurrent, respectively, and η is charge to mass ratio. The equation (1),which ignores the high order differential terms, is a first orderapproximation equation. Corresponding to different objective lenssystems, the radial deflection distance from Z-axis ω would express indifferent forms. For example, ω is a real constant in MOL, and it isrelated to the slope of the e-beam trajectory in SOL.

Due to the first order differential term exists, the distribution of thefocusing field, a compound field of electric and magnetic field, is anon-linear function. In order to satisfy the equation (1), D₁ and E₁ inthe right hand side have to give a non-linear field distributioncorresponding to a magnetic field determined by B and B′, and anelectric field determined by φ and φ′ in the left hand side,respectively. However, because of the non-linear property, it is quitedifficult to perform the distribution by deflectors and the objectivelens in practical. In the present, there are two mainly concepts used tooverwhelm the difficulty due to non-linear distribution. One is that thefirst order deflection field is separated to a linear field and anon-linear field, and the linear field is performed by a correspondingdevice and meanwhile the non-linear field is limited to be ignorable.For example, U.S. Pat. No. 6,717,144, which is illustrated in FIG. 2,disclosed a method of limiting the titled angle in a small range, so asto ignore the non-linear effect, and a compensation deflector 212 isarranged to compensate the aberration due to a rotation field, and ashift field is compensated by moving the stage 20, while an e-beam froman electron source 201 enters a deflector 210 and an objective lens 206.Therefore, the first order field distribution from the equation (1) canbe readily satisfied by the objective lens field and the twocompensation devices in common, without non-linear field distribution.The other keeps the adjustment of the parameters of the equation (1)until it is satisfied or approximately satisfied, namely, finding theoptimum solution. For instance, U.S. Pat. No. 6,392,231 disclosed theparameter to be adjusted, which is the radial deflection distance ω.However, in general, the optimum value is difficult to find, and whenthe e-beam kinetic energy is changed, the optimum value is greatlyimpacted so as to the off-aberrations increased.

To sum up, for the compound objective lens of focusing and deflectingfield, in particular to SEM with the compound objective lens and thelarger scanning field, it is still a big challenge to find a method toefficiently remove the off-axial aberrations.

Accordingly, the present invention has been devised to solve theabove-described problems. Specifically, the present invention provides amethod to determine a deflection field distribution which can readilyremove the off-axis aberrations in a swing objective lens of anelectro-optical system. Also, the present invention provides with amodification of the shape and the arrangement of the swing deflectionunit to achieve the non-linear distribution of the swing field. It isconsequence that a low off-axis aberration, high throughputelectro-optical tool is obtained.

A SEM of the present embodiment mainly includes a charged particlesource, an accelerating electrode for accelerating an e-beam, anobjective lens which is arranged above the stage and in rotationalsymmetry about an optical axis, for focusing the e-beam on a specimensurface, a decelerating lens formed by a potential different between thespecimen and the specimen-side end of the objective lens to decrease ane-beam landing energy (LE), and a deflection units system including atleast one deflection unit arranged along the optical axis in the bore ofthe objective lens for deflecting the e-beam and achieving a scan in alarge specimen region. Therein, the objective lens can be anelectrostatic lens, a magnetic lens, and a compound lens consisting ofmagnetic lenses and electric lenses, and at least one of the deflectionunits, which can be electric or magnetic deflectors, has to be disposedin the decelerating lens.

The method of a determination of the focusing and swing fielddistribution to approximately satisfy a first-order e-beam trajectoryequation of SOL in the present invention is described as following. Theequation includes a first focusing field, a second focusing field, and aswing field. Therein, the swing field generated by a swing deflectionunit is determined by the combination of the first and second focusingfield. In order to make the swing field easily be generated by the swingdeflection unit, and meanwhile, match the compound field, the presentinvention discloses that the compound field has to be limited in acertain range which can be provided by the swing deflection unit.Preferably, the first and second focusing fields are overlapped by eachother at least 80 percent. More preferably, each space position of eachmaximum of the first and second focusing field is closed to each other.The optimum is that both of the focusing fields have similar spacedistribution patterns of field intensities in the same region. Takingthe SOL with a compound lens as an example, a first-order electrostaticsfiled acting as its first focusing field and a first-orderpotential-magnetic coupling field acting as its second focusing field,and each maximum of them is located on the specimen or its adjacentarea. As a result, their compound field will be separated into twoparts; one is beneath the specimen and the other is above the specimen.Therefore, the above-part field is concerned to satisfy the equation (1)and the below-part field can be ignored. In a preferred embodiment, theswing deflection unit is a single deflector without other deflectors.

Regarding to achieve both of the maximums of the first and second fieldare located on or near the specimen surface, and meanwhile, theabove-part field generated by the swing deflection unit can satisfy theequation (1), the present invention discloses a means as the following.The modification of the shape, size and the arrangement of the swingdeflection unit are used to satisfy the above-mentioned demand under thefixed objective lens field. In other words, the space distribution ofthe swing field above the specimen can match the above-part field tosatisfy the equation (1).

Accordingly, the present invention provides a distribution which can bereadily generated by a swing deflection unit and be satisfied with thefirst-order e-beam trajectory equation of SOL, by modifying a shape andan arrangement of a swing deflection unit.

Various example embodiments of the present invention will now bedescribed more fully with reference to the accompanying drawings inwhich some example embodiments of the invention are shown. Withoutlimiting the scope of the protection of the present invention, all thedescription and drawings of the embodiments will exemplarily be referredto an electron beam. However, the embodiments are not be used to limitthe present invention to specific charged particles.

Please refer to FIG. 3a , it is a first embodiment of the presentinvention, which illustrates a SEM including a charged particle source301, an accelerating electrode 302, a condenser lens 303 and a swingobjective retarding immersion lens (SORIL) column. The column comprisesfour electrostatic deflectors 310, 312, 314 and 316, a magneticimmersion lens 306 including a coil 3061 surrounded by a high magneticpermeability yoke 3062 (including a pole piece), which immerses aspecimen 307 by its magnetic field, and a decelerating lens (i.e., aretarding electrode) 318 with a rotational symmetry about an opticalaxis 300, which is arranged between a specimen-side end of the objectivelens 306 and the specimen 307. Therein, said four deflectors areseparated into a first deflection group of deflectors 310 and 316 fordeflecting an e-beam and swinging an objective lens field, and a seconddeflection group of deflectors 312 and 314 for scanning the specimen307. For the first deflection deflector, the deflector 310 is an in-lenspre-deflector disposed in the bore of the objective lens 306 to generatea deflection field for deflecting the e-beam, and a swing deflector 316disposed near a gap of the yoke 3062 to provide a swing field forswinging the objective lens field and limiting the off-aberrations atthe same time. In a preferred embodiment, each of all said electrodes isan annular deflector including 12 electrode-plates, and each of theplates is connected to a variable voltage driver, such that the e-beamis controlled to be deflected in X and Y directions. The gap of the yoke3062 is poisoned near the specimen-side end of the objective lens 306,and the objective lens magnetic flux leakages from it. In the presentembodiment, the SORIL system has a short working distance so as to gainthe high resolution for the reduction of the space charged effect.

In the operation of the above SEM configuration, a high voltage (˜12KeV) is applied to the emitter 301 to emanate the high kinetic energye-beam. While the e-beam enters the SORIL, the pre-deflector 310 is usedto deflect the e-beam to a pre-determined direction, and then, thedeflected e-beam is focused and retarded by the objective lens 306 andretarding electrode 318 respectively before hitting the specimen 307.Therein, the retardation in the e-beam can avoid the damage to thespecimen 307 by such a high energy e-beam. In the example, the swingdeflector 316, which is arranged in the retarding field, has a greatereffect on the limitation of the off-aberrations than other deflectorsdo, because it acts on the decelerated e-beam and is much closed to thespecimen 307. That makes it possible to limit the off-aberrations byswing deflectors, even by one deflector as shown in the presentinvention. The first deflection group of deflectors 310 and 316 canposition the e-beam and move the e-beam in a large deflection distance(larger than 600 μm). And the second deflection group of deflectors 31and 314 moves the e-beam in a small region to rapidly scan the specimen307. By the two groups, the SORIL scan can be accomplished. After that,the secondary electrons (SE) and the back-scattered electrons (BSE) areemanated from the specimen 307 and received by a detector 308 to convertthe electronic signal into the image data. Moreover, the large beamcurrent is employed herein to improve the throughput, for the reductionof the e-beam residence time on the specimen.

As to establish the SORIL, the following electron trajectory equation(2) has to be satisfied:−VE ₁ =k[φ′(z)+½φ″(z)(z−z ₀)]−i√{square root over(2φ(z)ηk)}[B(z)+½B′(z)(z−z ₀)]  (2)

wherein V=V_(x)+iV_(y) represents the voltage of the swing deflector312, E₁ represents the swing field, B(z) and φ(z) represent the magneticand electric field distribution along Z-axis, respectively, B′(z) andφ′(z) denote the first order differentiation with respect to z, η is thecharge-to-mass ratio, z₀ is the position of objective plane, k is theslope of the swing optical axis. Therein, the first-order approximateelectric field [φ′(z)+½φ″(z)(z−z₀)] is a first focusing field and thefirst-order approximate coupling field √{square root over(φ(z))}[B(z)+½B′(z)(z−z₀)] is a second focusing field.

From equation (2), the deflection field E₁ on its left hand side, is thesuperimposition of √{square root over (φ(z))}[B(z)+½B′(z) (z−z₀)] and[φ+(z)+½φ″(z)(z−z₀)]. In general, both of the electric and magneticswing deflectors are used to provide a compound field for making the twosides of the equation (2) be equal. In other words, the elimination ofthe off-aberrations from SORIL is performed by at least two deflectors.

Moreover, in order to reduce the structure and product complexity, thepresent invention provides a SOL system which only needs one swingdeflector to satisfy the equation (2) and meanwhile, the off-aberrationsare limited. A simple and easy way to be generated field distribution isgiven to satisfy the equation (2) through a single swing deflector.However, the above distribution is not achievable as the superimpositionfield is an irregular distribution.

Please refer to FIG. 3b , a preferred distribution is provided by theembodiment in accordance with the present invention. The two focusingfields are overlapped by each other at least 80%, as shown in the hatcharea of FIG. 3b . In this case, the superimposition field distributionof the two focusing fields can form an approximate smooth curve with asingle peak and two sides of the peak are strictly monotonic curves.More preferably, each maximum of the first and second focusing field isclosed to each other at the space position, as shown in FIG. 3c . Theoptimum is that both of the focusing fields have similar spacedistribution patterns of field intensities in the same region, andmeanwhile, their maximums are located on the specimen or its adjacentarea, as shown in FIG. 3b . As a result, in the optimum case, onlysingle swing deflector is needed to generate a swing field which canlimit the SORIL off-aberrations. Therein, the two focusing fields canmatch with each other, as the described in the optimum case, bysatisfying the following equation (3):[φ′(z)+½φ″(z)(z−z ₀)]=√{square root over (φ(z))}[B(z)+½B′(z)(z−z₀)]  (3)wherein φ′(z) represents the electric field distribution along Z-axis,which is determined by the swing deflector 312. While the two sides ofthe equation (3) are equal, it is easy to satisfy the equation (2).Please refer to FIG. 3e , the modification of the shape, size and thearrangement of the swing deflector 316 and of the position of themagnetic lens 306 are used to accomplish SORIL under the fixed currentand voltage supplied to the compound lens of B(z) and φ′(z) in equation(2). B(z), which is leaked from the magnetic lens 306, can be determinedby the adjustment of the distance, C_(i), between an inner pole piece ofthe magnetic lens and a specimen 307, and of the bore size, D₁, of theinner pole piece. On the other hands, the modification of the distance,C₂, between the swing deflector 316 and the specimen 307, and the boresize, D₂, of the swing deflector 316, and of the shape of the swingdeflector 316 are used to provide a swing filed which can match thecompound field. Therein, the shape of the swing deflector can be thatits upper inner-face is tilted an angle to the outer of the SEM to forma deviation deflector. In the case, the scanning angle of the e-beam isalso extended from 1 to15 degrees.

In another aspect, the swing deflector 316 is not limited to a singledeflector in the present embodiment. As the distribution position of themaximum of B(z) and/or φ′(z) is not limited on the adjacent area of thespecimen surface, their compound field distribution would become morecomplicated, so as to require more than one swing deflector to satisfythe equation (2). For example, referring to FIG. 3f , the distributionsof two focusing fields change from positive to negative above thespecimen surface (i e , image plane), and only one deflector cannotprovide the non-single peak distribution, so that more than onedeflectors are required to satisfy the distributions.

A second embodiment of the present invention is illustrated in FIG. 4,which discloses a SEM comprises a charged particle source 401, anaccelerating electrode 402, a pre-condenser lens 403, a detector 408,and a swing objective lens system including a pre-deflection units 410,a swing deflection units 416, and an objective lens 406, all of them arerotationally symmetric arrangement with respect to an optical axis 400.Therein, the objective lens 406 can be a coil, an electrode, or a coilsurrounding by a yoke. Preferably, the objective lens 406 provides animmersion field to the specimen 407.

Accordingly, the distributions of B(z) and φ′(z) of the equation (2)follow that of the first embodiment and satisfy the equation (3), so asto use only one swing deflection unit to achieve a swing objective lens.And referring to the first embodiment, the swing field of the swingdeflection units 416, which matches the distributions of B(z) and φ′(z),can be obtained by adjusting the following parameters: firstly, thedistance between an inner pole piece of the objective lens 406 and aspecimen 407, and the bore size of the inner pole piece, and then, thedistance between the specimen-side end of the swing deflection units 416and a specimen 407, and the bore size of the swing deflection units 416,and finally, the shape of the swing deflection unit 416, which is thatits upper inner-face is tilted an angle θ to the outer of the SEM and alower inner-face parallel to the optical axis 400.

Although the present invention has been described in accordance with theembodiments shown, one of ordinary skill in the art will readilyrecognize that there could be variations to the embodiments and thosevariations would be within the spirit and scope of the presentinvention. Accordingly, many modifications may be made by one ofordinary skill in the art without departing from the spirit and scope ofthe appended claims.

The following description presents the main features difference betweenthe present invention and the prior arts. The first prior art, Lowenergy large scan field electron beam column for wafer inspection (byLiu et al.), and the second prior art, U.S. Pat. No. 6,392,231, providea SORIL system, which have an immersion objective lens, five deflectors,one of them is located below the objective lens, and a retarding lensarranged near the specimen. The equation (1) is used herein to establisha SORIL. The third prior art, U.S. Pat. No. 6,627,890, disclosed amulti-poles correction device arranged under the objective lens tocorrect the off-aberrations. The fourth prior art, E.P. Pat. No.2,378,537, provided an aberration correction device, which included anoptical correction device and a movable charged particle beam tiltingdevice, arranged in the objective lens to correct the objective lensaberration by controlling the beam aperture angle. The fifth prior art,U.S. Pat. No. 6,534,766, to Toshiba Ltd. and Topcon Ltd. disclose adeflector arranged under the objective lens to compensate theaberrations. However, its working distance is too long to furtherimprove the space resolution. The sixth to ninth prior arts, U.S. Pat.Nos. 6,452,175, 6,825,475, 6,380,546, and U.S. Pat. No. 7,112,803, toApplied Materials Ltd. provide a first deflector disposed below anon-immersion objective lens, which was thin to get a short workingdistance, in common with a second deflector for compensating theaberrations. The tenth prior art, U.S. Pat. No. 6,747,279, discloses amixed configuration of an objective lens and a deflector so that theworking distance would not increase for higher resolution.

According to the above-mentioned recitations, the main differencebetween the present invention and the prior arts is that thesuperimposition field distribution of the first and the second focusingfields in the objective lens can be provided by only one swingdeflection unit. Therein, preferably, the first and second focusingfields are overlapped by each other at least 80 percent. Morepreferably, each space position of each maximum of the first and secondfocusing field is closed to each other. The optimum is that both of thefocusing fields have similar space distribution patterns of fieldintensities in the same region. And the means to achieve the demand isthat upper inner-face of the swing deflection unit is tilted at an angleθ to the outer of the SEM and its lower inner-face is parallel to theoptical axis. The third to tenth prior arts do not define the shape ofthe swing deflector and ignore the first-order field terms in SOLequation. The first and second prior arts have considered thefirst-order field terms. However, they do not design the shape of theswing deflection unit. Therefore, the present invention cannot be taughtby the above prior arts.

What is claimed is:
 1. A system for tilting a charged particle beamfocused by an immersion objective lens, comprising: a pre-lens deflectorabove and adjacent to a magnetic field generated by the immersionobjective lens, said pre-lens deflector deflecting the charged particlebeam to a pre-determined angle; and a swing deflector, located insidesaid immersion objective lens, for swinging a magnetic lens generated bythe immersion objective lens into the pre-determined angle, such thatthe charged particle beam is focused by the swung magnetic lens andbombards a specimen with the pre-determined angle.
 2. The systemaccording to claim 1, wherein the charged particle beam is an electronbeam.
 3. A swing objective lens, comprising: an immersion objective lensfor focusing a charged particle beam on a specimen; a pre-lens deflectorabove and adjacent to a magnetic lens generated by the immersionobjective lens, said pre-lens deflecting the charged particle beam to apre-determined angle; and a swing deflector, located inside saidmagnetic lens, providing an electrostatic field with a first condition;wherein when the first condition matches a second condition generated bythe immersion objective lens and the swing deflector, the magnetic lensis swung into the pre-determined angle.
 4. The swing objective lensaccording to claim 3, wherein the first condition is[φ′(z)+½″(z)(z−z₀)], and φ is an electric potential produced by theswing deflector.
 5. The swing objective lens according to claim 4,wherein the second condition is √{square root over(φ(z))}[B(z)+½B′(z)(z−z₀)], and B is the magnetic field generated by theimmersion objective lens.
 6. The swing objective lens according to claim5, wherein the match is the first condition mostly overlapped with thesecond condition.
 7. The swing objective lens according to claim 5,wherein the match is a first peak value of the first condition close toa second peak value of the second condition.
 8. The swing objective lensaccording to claim 5, wherein the match is a first distribution of thefirst condition similar and close to a second distribution of the secondcondition.
 9. A swing objective lens, comprising: an immersion objectivelens for focusing a charged particle beam on a specimen, wherein thecharged particle beam along an optical axis; a pre-lens deflector aboveand adjacent to a magnetic lens generated by the immersion objectivelens, said pre-lens deflecting the charged particle beam to apre-determined angle; and a swing deflector, located inside saidmagnetic lens, a first portion of an inner surface of the swingdeflector having an inclined angle to the optical axis and a secondportion of the inner surface of the swing deflector being parallel tothe optical axis, wherein the magnetic lens is swung by the swingdeflector into the pre-determined angle.
 10. The swing objective lensaccording to claim 9, wherein the swing deflector generates an electricpotential φ with a first condition, [φ′(z)+½φ″(z)(z−z₀)].
 11. The swingobjective lens according to claim 10, wherein the immersion objectivelens generates a magnetic field B with a second condition, √{square rootover (φ(z))}[B(z)+½′(z)(z−z₀)].
 12. The swing objective lens accordingto claim 11, wherein the first condition matches the second condition.13. The swing objective lens according to claim 12, wherein the match isa first distribution of the first condition similar and close to asecond distribution of the second condition.
 14. The swing objectivelens according to claim 13, further comprising a scanning deflector unitfor scanning the charged particle beam on the specimen.
 15. The swingobjective lens according to claim 14, further comprising a retardelectrode below the immersion objective lens.
 16. A scanning electronmicroscope, comprising: an electron source for providing an electronbeam along an optical axis; a condenser lens for condensing the electronbeam; a detector for receiving signal electrons emanating from thespecimen; and a swing objective lens, comprising: an immersion objectivelens for focusing a charged particle beam on a specimen, wherein thecharged particle beam along an optical axis, wherein the immersionobjective lens generates a magnetic field B with a second condition,√{square root over (φ(z))}[B(z)+½B′(z)(z−z₀)]; a pre-lens deflectorabove and adjacent to a magnetic lens generated by the immersionobjective lens, said pre-lens deflecting the charged particle beam to apre-determined angle; a swing deflector, located inside said magneticlens, a first portion of an inner surface of the swing deflector havingan inclined angle to the optical axis and a second portion of the innersurface of the swing deflector being parallel to the optical axis,wherein the magnetic lens is swung by the swing deflector into thepre-determined angle, the swing deflector generates an electricpotential co with a first condition, [φ′(z)+½φ″(z)(z−z₀)], and a firstdistribution of the first condition similar and close to a seconddistribution of the second condition; a scanning deflector unit forscanning the charged particle beam on the specimen.; and a retardelectrode below the immersion objective lens.
 17. A method for tilting acharged particle beam, comprising: deflecting the charged particle beamto a pre-determined angle; providing an immersion magnetic lens to thecharged particle beam such that the charged particle beam is focused ona specimen; and providing a swinging electrostatic field to theimmersion magnetic lens such that the immersion magnetic lens is swungto the pre-determined angle, wherein the swing electric field has afirst condition match to a second condition generated by the immersionmagnetic lens and the swinging electrostatic field.